Method of adhesively assembling plastic products

ABSTRACT

A method for adhesively assembling plastic products which comprises placing adjacent to the surfaces of the pieces to be joined a continuous, non-perforated metallic foil strip, which absorbs electromagnetic waves. The strip, which is contiguous with a heat activatable adhesive material, is positioned between the pieces and exposed to electromagnetic waves while the pieces are held together to form a bonded relationship between the plastic pieces.

CROSS REFERENCE TO A RELATED APPLICATION

This application is a continuation-in-part of U.S. Patent ApplicationSer. No. 08/689,188 filed on Aug. 5, 1996 now abandoned.

BACKGROUND OF THE INVENTION

This disclosure relates to the assembly of a variety of plastic productsusing an adhesive device that is activated by electromagnetic waves. Theplastic products include assemblies from such materials such assynthetic fabrics such as vinyls, rayon, nylon; and items such asplastic toys, picture frames, plastic auto and RV parts and componentsand the like.

The assembly of plastics is typically effected by adhesives, stitching,stapling; and, in the case of plastic fabrics, heat staking andultrasonic welding. Each method has its strengths and weaknessesdepending on the application.

However, these traditional adhesives are rapidly falling into disfavoras it becomes desirable and even necessary to work quicker and neaterand to develop even more secure adhesive bonds. It is apparent, then,that inventions are waiting to be made which address the placement ofadhesive material in a neat, clean, safe and effective manner especiallywith regard to the assembly of plastic products.

Not surprisingly then, others have experimented with alternatives totraditional fastening devices for adhesively joining the components ofan assembled product.

DESCRIPTION OF THE PRIOR ART

U.S. Pat. No. 4,038,120 to Russell describes the use of an energizedheating element or wire to heat a hot melt glue resulting in adhesionbetween contiguously assembled panels. The reference method involvesheating a glue-coated wire to liquefy the glue producing a cohesivestate and facilitating the assembly of panels. This method isparticularly useful for introducing a cohesive material (glue) to anarea of limited accessibility (groove), but the heating element (wire)requires the direct application of energy (electricity) to provide theheat to melt glue.

U.S. Pat. No. 3,574,031 to Heller et al. describes a method and materialfor welding thermoplastic bodies by using a susceptor between the bodiesto be joined. The susceptor sealant is characterized by havingparticles, heatable by induction, dielectric or radiant energy,dispersed in a thermoplastic carrier compatible with the thermoplasticsheets to be welded. The welding of the thermoplastic sheets is effectedby exposing the susceptor sealant to heat energy, softening the carriermaterial and joining all thermoplastic materials.

U.S. Pat. No. 3,996,402 to Sindt relates to the assembly of sheetmaterials by the use of a fastening device utilizing an apertured sheetof eddy current-conducting material sandwiched between coatings ofhot-melt glue. An induction heating system is activated causing eddycurrent heating in the EC-conducting material with consequent melting ofthe hot-melt glue thus resulting in fusion and, ultimately, bonding ofthe sheet materials in accordance with the desired construction.

SUMMARY OF THE INVENTION

The presently disclosed method of adhesively adhering component piecesof an assembled plastic product is distinguished from, and improvesupon, the prior art by utilizing a device to be placed adjacent to thesurfaces to be joined which comprises a target element contiguous with aheat activatable adhesive material said target element being absorbentof electromagnetic waves which are convertible to heat energy foractivating the adhesive material, holding said surfaces together, andexposing said device to electromagnetic waves to produce heat sufficientto activate the adhesive material to effect an adhesive bond between thecomponent pieces of the assembled plastic product.

DESCRIPTION OF THE PREFERRED EMBODIMENT

It is imperative that the assembly of simple plastic products beconstructed as efficiently as possible. There is not a lot of technologyor know-how in the fabrication of these items. Typically, they're madeof low grade plastics fastened by traditional fasteners such as screws,nuts, bolts, heat staking, ultrasonic welding, stitching, stapling,adhesives and hot melts. It is believed that the disclosed method ofassembling plastic products will provide definite advantages of cost,safety, and appearance over traditionally fastened plastic products.

By the term "plastic products" is meant all organic polymers that aremoldable and extrudable to assume an infinite variety of forms andshapes so as to perform the innumerable functions observed andenvisioned for these versatile materials.

Also, bear in mind that these plastic products are characteristicallymade in assembly line operations. They're made with standard piecesaccording to standard specifications. This uniformity of composition andprocess is well suited for the employment of an adhesive device that canbe desirably situated between or adjacent to pieces to be joined in theassembly of the product and activated to adhesively join the componentsinto a sturdy, well-constructed product.

Looking at the adhesive device employed in the disclosed method ingreater detail, we see that the target element must, for the most part,be fashioned from materials or substances that are not transparent toelectromagnetic waves. Indeed, the target element will necessarily beconstructed of a composition that will absorb electromagnetic waves.Once absorbed by the target element, these waves will produce magnetichysteresis and eddy currents resulting in heat energy which will melt oractivate the contiguous adhesive material.

Typically, the target element will be fashioned from metallic materialssuch as steel, aluminum, copper, nickel or amalgams thereof which haveproven utility and are readily available; although, some semimetallicmaterials such as carbon and silicon are also known to be suitable forthe absorption of electromagnetic waves.

The target element can assume any form or shape consistent with theoverall configuration of the adhesive device. Frequently, the targetelement will be presented as a metallic foil, strip, and, in someinstances, it will be more effective to present the target element inthe form of a fiber of an electromagnetic absorbable material. The pointto be made is that the target element need only be fashioned from amaterial reasonably impervious to, and absorptive of, electromagneticwaves.

In use, the adhesive device needs to be situated adjacent to the piecesor components of the plastic product to be assembled. As a practicalmatter, of course, all plastic products are transparent toelectromagnetic waves. Some plastic materials will be more transparentthan others, and empirical adjustments can and will be made to modulatethe quantity and intensity of electromagnetic wave energy needed tooptimally activate the adhesive material.

In many instances, it will be sufficient for the adhesive device simplyto be placed adjacent to the plastic product pieces to be assembled. Inother construction or assembly situations, it will be necessary to makesome arrangements or take additional steps to make sure the adhesivedevice remains in place prior to activation. Such an additional stepneed be little more than introducing an attachment element such as asmall pressure sensitive adhesive area on the surface of the device.Simpler means for positioning the device prior to activation mightentail clamping, tacking, stapling or spiking to make sure the adhesivedevice is situated and activated in the most effective and, therefore,most desirable location. But these measures, of course, would beoptional procedures and in no way essential to the performance of thedevice in its broadest typical and routine applications.

When desirably situated adjacent to the plastic components of theproduct to be assembled, the adhesive device is ready to be exposed toelectromagnetic waves, produced by and emanating from a generatorpowered by a source of alternating electric current. The generator canbe held in a fixed position for assembly-line production or designed tobe manipulated so as to quickly and easily pass over, around or near thestrategically "hidden" device while emitting electromagnetic waves whichwill penetrate the "transparent" plastic components to be assembled, beabsorbed by the target element, be converted to heat energy, activatethe adhesive material resulting in a bonded relationship between thepieces of the plastic product to be assembled.

To elaborate, somewhat, heat is produced in the conductive targetelement by two mechanisms: eddy current resistive heating and magnetichysteresis. Eddy current resistive heating applies to all conductivematerials and is produced in the target element by the electromagneticwaves emanating from the generator. The heat resulting from magnetichysteresis is observed only in magnetic materials. As theelectromagnetic field produced by the generator reverses polarity, themagnetized atoms or molecules in the target element also reverse. Thereis an energy loss in this reversal which is analogous to friction: thisenergy loss is magnetic hysteresis. The "lost" energy is quicklyconverted to heat and conducted by the target material to thecontiguous, and frequently enveloping, heat-activatable adhesivematerial to initiate adhesion.

While the aforementioned heating mechanisms apply to most forms ofabsorbent target materials, there are factors which favor the use of acontinuous, non-perforated metallic foil. These factors make foiltargets having no apertures heat faster, more efficiently, and saferthan other forms.

In the eddy current resistive heating mechanism, the foil presents alarger target area; thus, more of the EM field is absorbed when comparedto either particles or mesh. Thus, for a given EM field strength, thefoil target heats more rapidly. Also, the foil allows the eddy currentsto have an unobstructed current loop path. Meanwhile, particles areeffectively unbeatable by eddy currents since the gaps between particlesdo not allow a current loop path. In meshes, the current loop path isdisrupted by the mesh which has the effect of regional uneven heatingand localized hot spots.

In the magnetic hysteresis heating mechanism, the target must be formedof magnetically susceptible materials such as iron, nickel, cobalt, andcompounds containing these elements. Magnetic hysteresis takes placeeach time the EM field reverses, thus higher heating rates are observedat higher frequency. Adhesives which are loaded with magneticallysusceptible powders are generally heated at or above 10 megahertz.

The use of a foil target material allows a weaker EM field at a lowerfrequency than either meshes or particles. This yields several benefits.The EM field generator is smaller, lighter, and requires lower inputenergy. The lower frequency is also safer and allows operation withoutspecial guarding or other safety provisions. The preferred range forthis invention is 50 kilohertz to 900 kilohertz; ideally, the frequencyrange is between 150 kilohertz and 300 kilohertz. The IEEE (Institute ofElectrical and Electronic Engineers) standard C95.1-1991 refers to humansafety for electromagnetic field exposure. This standard has also beenadopted by the ACGIH (American Conference of Governmental and IndustrialHygenists) for "Biological Exposure Indices" 1996.

When heated to the necessary temperature, the adhesive material willliquefy or become heat-activated, attach itself to the surfaces to bejoined and, on cooling, create an adhesive relationship between thejoined plastic components of the assembled product.

Two adhesion mechanisms, hot-melt and heat-activated cure, are proposedfor use with the disclosed device. Both mechanisms are initiated by heatemanating from the target element. Hot-melt adhesives are solid atambient temperatures, but melt or liquefy when the temperature iselevated by, for instance, heat accumulating in the target element. Themelted adhesive "wets" the adherends and attaches to the surface of thepieces to be bonded. As the adhesive cools, the adherends and adhesiveare bonded by the electrostatic attraction of polar molecular groups.Note that for the hot-melt mechanism, the bonding is reversible. Thus byrepeating the induction heating procedure, the bond can be undone andthe adherends separated. The ability to reverse the adhesion andseparate adhesively assembled pieces is not a trivial attribute. Inaddition to the obvious advantage of being able to reassemble or repairmisaligned pieces in assembled products, it may also be desirable to beable to disassemble adhesively assembled products to facilitateserviceability and repair.

Heat-activated curing adhesives are also solid and easy to manipulate atambient temperatures, but when the adhesive temperature is elevated by,for example, the heat emanating from the target element, a chemicalreaction is initiated. This reaction involves a cure or crosslinkedbonding either within the adhesive or between the adherends. Such bondsare typically irreversible. Frequently, a heat-activated curing adhesivebond will demonstrate an electrostatic attraction between the adhesiveand the adherends and a crosslinked bond within itself.

While the foregoing is a complete description of the disclosed method,numerous variations and modifications may also be employed to implementthe purpose of the invention. And, therefore, the elaboration providedshould not be assumed to limit the scope of the invention which isintended to be defined by the appended claims.

What is claimed is:
 1. A method of assembling plastic products, whichcomprises:placing adjacent to the surfaces of plastic pieces to bejoined a device comprising a target element composed of a continuous,non-perforated metallic foil strip contiguous with a heat activatableadhesive material, said target material being absorbent ofelectromagnetic waves which are convertible to heat energy to activatesaid adhesive material, holding said surfaces together, and exposingsaid device to electromagnetic waves to produce heat sufficient toactivate the adhesive material to effect a bonded relationship betweenthe plastic pieces.
 2. The method of claim 1, wherein the frequency ofthe electromagnetic waves is between 50 kilohertz and 900 kilohertz. 3.The method of claim 1, wherein said foil strip is non-magnetic.
 4. Themethod of claim 1, wherein said foil strip is manufactured from ametallic material taken from a group consisting of aluminum, copper, andsteel.
 5. The method of claim 1, wherein said heat activatable adhesivematerial is a hot-melt adhesive.
 6. The method of claim 1, wherein saidheat activatable adhesive material is a heat-activated curing adhesive.7. The method of claim 1, wherein said heat energy is generated by eddycurrents.
 8. The method of claim 1, wherein said heat energy isgenerated by hysteresis.
 9. The method of claim 1, wherein the frequencyof the electromagnetic waves is between 150 kilohertz and 350 kilohertz.10. A method of assembling plastic products, which comprises:placingadjacent to the surfaces of plastic pieces to be joined a devicecomprising a target element composed of a metallic foil strip contiguouswith a heat activatable adhesive material, said target material beingabsorbent of electromagnetic waves which are convertible to heat energyto activate said adhesive material, holding said surfaces together, andexposing said device to electromagnetic waves having frequency between150 kilohertz and 300 kilohertz to produce heat sufficient to activatethe adhesive material to effect a bonded relationship between theplastic pieces.